In a rotating electrical machine, such as an automobile and a motor for general industries, a demand has grown for high output and size reduction with high density. With regard to the size reduction, insulated wires are made into the cross-section rectangular form, in order to improve conductor occupancy in a slot of a stator core of the rotating electrical machine. In recent years, in the rotating electrical machine, the insulated wire is cut and turn-processed into U-shape or the like, and two ends (open end portions) of the U-shape or the like of a plurality of the insulated wires having U-shape or the like are alternately connected into a coil, and set in the slots of the stator core. This coil (insulated wire divided segments) of the thus connected insulated wire having U shape or the like is constituted of: a slot housing portion (slot straight-line portion) to be housed in the slot; and a coil end portion that is not housed in the slot (a turn portion having U shape or the like, and open end portions being two ends). Two linear slot housing portions of the coil of the insulated wire having U-shape or the like are set in different slots, respectively. On the other hand, curved coil end portions having U-shape or the like, and coil end portions in the open end portions are aligned in a state in which both are not housed in the slots, and protruded therefrom in parts above and below the slots, and the coil end portions in the open end portions and other coil open end portions are connected and wired. Thus, achievement of higher density is attempted, by further shortening the coil end portions.
On the other hand, high output is achieved, by achievement of high voltage, high current, and high rotation in the rotating electrical machine.
Herein, the coil is formed by bundling a plurality of insulation-covered conductors vertically, horizontally, or vertically and horizontally, and when necessary, the plurality of these bundled insulated wires are wholly covered with resin for electric field relaxation or achievement of insulation.
However, at a conventional high voltage exceeding a specified voltage at several of kHz to several tens of kHz, insulation performance has been insufficient by the coil or the insulated wire to be used in the rotating electrical machine. In particular, durability in a long-term use is required.
With regard to this durability, corona discharge is generated among coated conductors (among a plurality of bundled coated conductors) at predetermined electric field intensity or more, and the durability is adversely affected. It is strongly required to suppress this corona discharge, namely partial discharge deterioration to minimum.
In general, the partial discharge deterioration is a phenomenon in which an insulating material complexly receives molecular chain scission deterioration by collision of charged particles generated by partial discharge thereof, sputtering deterioration, thermal fusion or thermal decomposition deterioration by a local temperature rise, chemical deterioration by ozone generated by electric discharge, and the like. Thus, thickness is reduced, in several cases, in the insulating material that is deteriorated by actual partial discharge.
This partial discharge deterioration is apt to cause at the curved coil end portion having U-shape or the like, which is neither housed nor fixed in the slot. For example, proposals have been made on varying: a thickness of an insulation coating with which a conductor is coated; and an insulating material with which the conductor is coated, in the above-described slot housing portions and the coil end portions of the coils of insulated wires (see Patent Literature 1).